Picture used for illustrative purpose only.
Exhaustive testing by fire and hazardous material teams found no toxic material at Facebook Inc's Silicon Valley campus after fears that a package at its mail facility contained the nerve agent sarin.
The campus was given all clear by the authorities.
The social media company had to evacuate four buildings and two people were checked for possible exposure to the compound that attacks the nervous system and can be fatal, said Jon Johnston, fire marshal for the city of Menlo Park in California where Facebook is based.
"There is no sarin," Johnston said, referring to the package that had erroneously tested positive on Monday morning.
Facebook routinely checks all packages and had initiated a standard safety protocol, Johnston added, saying teams worked into the early hours of Tuesday to clear the scene.
Federal Bureau of Investigation (FBI) agents also went to the scene, Facebook said. A Facebook spokesman confirmed the all-clear on Tuesday.
"Authorities have confirmed test results were negative for any potentially dangerous substance and the buildings have been cleared for repopulation," said Anthony Harrison, Facebook's director of corporate media relations.
With 2.3 billion monthly active users worldwide and more than $55 billion in revenue in 2018, Facebook faces criticism for its control of personal information and has been subject to cyber attacks.
If Facebook continues to expand at current rates, the number of deceased users could reach as high as 4.9 billion before the end of the century, making it the world's biggest graveyard, predict researchers from the University of Oxford.
The formal investigation opens a new chapter in the European Union's campaign to address the dominance of US tech firms with Google, Facebook and Apple also regular targets of regulators in Brussels.
Facebook has removed 2,632 Pages, Groups and accounts that were engaged in coordinated inauthentic behaviour from Iran, Russia, Macedonia and Kosovo on its platform, as well as on Instagram.
The French Council of the Muslim Faith (CFCM) said on Monday it was suing internet giants Facebook and YouTube for allowing the public broadcast of a live video by the man who carried out the New Zealand mosque massacre this month.
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